Method and apparatus for monitoring low concentrations of ions in a flowing fluid

ABSTRACT

A method and apparatus for monitoring low concentrations of certain ions present in a flowing fluid is described in which an isokinetic sample is passed through an electroseparative cell wherein, under the influence of an applied electrical field microgram quantities of the ionic species are quantitatively transferred through a microporous filter membrane. The membrane has a pore size related to cell efficiency.

BACKGROUND OF THE INVENTION

This invention relates to analytical apparatus and the methods foranalytical monitoring of liquids, and further relates to the preparationof water in a condition free of ions of a certain selected species.

A requirement which frequently arises in relation to plant using ortreating certain liquids is that the plant operator should know or havemeans for ascertaining the concentration of chemical species present inthe liquid being treated or processed. There may be many reasons why theneed for this information arises, for example the need for qualitycontrol of a particular product, or the need for checking the presenceof corrosion-inducing species or the need for economy in the use ofexpensive additives to certain process liquids. Where the concentrationsof interest fall below, or at the lower end of, the measuring range ofconventional analytical instruments, difficulties arise in providing theplant operator with the data he needs and he remain unaware whethercorrective action is necessary. One such difficulty arises in themonitoring of corrosion and stress corrosion cracking of stainless steelcomponents by chloride species in high pressure boiler systems where itis important that any chlorides should be at a very low level. It is ourexperience that the chloride level in the feed water is already belowthe limit of detection by the on-stream colorimetric method and therequirement exists for monitoring even lower levels than are measurableby this means.

SUMMARY OF THE INVENTION

According to the present invention in one aspect an analytical apparatusfor detecting the presence of, or measuring, low concentrations of ionsof a selected species in solution in a liquid, includes anelectro-separative cell having a compartment for containing a specimenliquid substantially devoid of the selected species and a samplecompartment adapted to form part of a flowpath for a flowing isokineticsample of the liquid, a microporous neutral membrane separating thecompartments; an electrode in each compartment which is selectivelyenergised to attract the said ions from the flowing sample into thecompartment for containing the specimen and a means for measuring theconcentration of the ion species of interest accumulated in the specimencompartment.

Preferably electrodes of different polarity are supported one i.e., eachcompartment and the metal portions of the compartments ie the sample andspecimen inlet and outlet ports, are earthed. The specimen liquid may bea static volume of liquid, which is drained periodically and theconcentration of the species of interest present measured. Alternativelythe compartment for the specimen liquid may be adapted to form part of aflow path for a stream of specimen liquid substantially devoid of thespecies of interest. Means are then provided for directing the specimenoutlet flow through the measuring means.

It is of advantage to ensure that the specimen is made devoid of thespecies of interest by the same means as is employed to subsequentlyconcentrate said species in the specimen and electroseparation hasspecial merit in this respect. Hence the specimen, whether static orflowing is itself preferably the product of purification i.e., made freeof the ions of the species of interest by electro-separation.

The apparatus finds particular application in monitoring boiler feedwater for the presence of species in concentrations too low forconventional measurement yet liable to promote the onset of somedeleterious condition, such as corrosion. According to this aspect theinvention may comprise, in a steam boiler installation an analyticalapparatus for monitoring the presence of ions of said species which maybe present in solution in the boiler feed water the apparatus comprisingfirst and second similar electro-separative cells, -- each cell having asample compartment and a specimen compartment separated by a microporousmembrane and electrodes of opposite polarity in respective compartment,means for flowing a sample of boiler feed water through the samplecompartment of the first cell with the polarity of its electrodesarranged to attract species of interest through the micro-porousmembrane into the adjacent specimen compartment, said adjacentcompartment containing a static or flowing specimen of water madesubstantially devoid of said species by electro separative treatment insaid second cell and means for analysing the concentrated species in thespecimen compartment of the first cell. The source of said static orflowing specimen in the compartment of the first cell may be thecondensate well of the steam plant associated with the steam boiler. Insome cases the condensate may be passed directly to the firstelectroseparative cell and sample liquid may be recycled, afterpurification, e.g., by electrodialysis or other means, back to thespecimen compartment of the first cell. The invention also resides in amethod of monitoring a flowing liquid for the presence of ions of aselected species present in concentrations too low for routinemeasurement, the method residing in tapping an isokinetic sample from aflowing liquid, flowing the sample through a chamber bounded by one sideof a porous membrane whilst maintaining an electrical potential on theother side of the porous membrane sufficient to effect transport ofselected ions from the flowing liquid through the membrane into a volumeof liquid initially substantially free of said ions whereby said ionsbecome concentrated in the volume of liquid, continuing the isokineticsample flow until a concentration of ions sufficient for measurement hasbeen achieved and then measuring said concentration and relating thatmeasurement to the total flow of liquid.

It will be understood by those skilled in the art that the above processmay be employed for preparing blank water for analytical purposes.

DESCRIPTION OF THE DRAWINGS

In order that the invention may be better understood the analyticalapparatus, applications and methods will now be explained with referenceto the embodiments of the invention shown in the accompanyingdiagrammatic drawings in which

FIG. 1 shows an analytical apparatus for measuring low concentrations ina liquid

FIG. 2 shows a similar apparatus as applied to measure lowconcentrations of selected species in boiler feed water and

FIG. 3 shows graphically the relationship between various pore sizes ofthe microporous membrane and chloride concentration efficiency; the poresizes lying in the range 0.01 to 1.2 μm.

Referring firstly to FIG. 1, the reference 1 indicates a raw watersupply line supplying water via a treatment plant 2 through duct 3 to aprocess plant 4. The requirement is to measure the presence of a certainionisable specie (referred to herein as the specie(s) of interest) inthe water, in concentrations below that easily measurable byconventional plant instruments. To this end, a tapping pipe 5 from theduct 3 leads an isokinetic sample flow to an electro-separative cell 6.The sample passes through one (cathode) compartment 7 in the cellbounded on one side by a microporous membrane 8 and thence, via outlet9, to waste. The membrane 8 was a Millipore cellulose estor filtermembrane with holes about 0.45 μm diameter supplied by the Millipore Co.UK. The membrane was not ion-selective. On the other side of themembrane 8, a second compartment 10 contains as anolyte a static volumeof liquid from which the specie(s) of interest have been removed, e.g.,by electrodialysis, so that it is substantially devoid of saidspecie(s), or as devoid as it may be made by electrodialysis.

Each compartment has a plate electrode 11 of platinum arranged to beenergised by 100 volts dc supply in a sense to attract negative chlorideions from the sample flow through the membrane 8, which is porous, intothe specimen volume in compartment 10. Once the latter contains a higherconcentration of chloride ions, the content is drained via normallyclosed outlet 12 into pipe 14. The liquid is treated with reagent from asupply 15 and passed through a mixer 16. Having been mixed with achloride reagent, the mixture is passed to a colorimeter 17 wherein theconcentration of chloride species is measured. The chamber 10 is thenrecharged with a fresh volume of water devoid of the species of interestand the same cycle is repeated. Laboratory tests using radio-chlorinetracers have shown that the cell efficiency to be greater than 90% at achloride concentration of μg 1.sup.⁻¹ with a sample flow of 30 mlmin.sup.⁻¹, the isokinetic sample flow rate for a nuclear boilerchannel.

FIG. 2 shows a modified form of the apparatus in FIG. 1 as applied toanalyse boiler feed water for chloride species which are known to beresponsible for corrosion of the boiler internals.

So in FIG. 2, reference 21 indicates a water supply line leading to awater treatment plant 22, from which water preheated in feed heaters(not shown) is passed via main 23 to a steam boiler plant 24. Pertainingto the latter, a condensate well 25 is shown for reasons which willappear later.

In order to monitor the feed water for deleterious species such as, inthis case chlorides, an analysing apparatus is provided. The lattercomprises a pair of electrodialysis cells 26, 27 each of similarconstruction comprising a hollow body 28 divided into two compartments29, 30 by a microporous membrane 31. Platinum sheet electrodes 32 arepolarised in a sense chosen in cooperation with the selective nature ofthe solute to separate a given species, as will become apparent from thefollowing description.

A tapping pipe 34 leads a sample flow into inlet 35 at one end ofcompartment 29 bounded by the membrane 31. The sample flow runs thelength of the compartment 29 and allowed to run to waste from an outlet36 at the other end of the compartment. Simultaneously, a flow ofspecimen water devoid of the species of interest is flowed incountercurrent to the sample flow into inlet 37 into the compartment 30of cell 26 through the compartment 30 and out through outlet 38 intopipe 39. Flow through compartment 29 is arranged to be ten times theflow through compartment 30 in order that a ten fold concentrationeffect may be achieved. It will be understood that, for the analysis ofchlorides, the electrode 32 in compartment 30 is given a positivepotential relative to that in compartment 29 so that, with themicroporous membrane 31 anion permeable, any chloride ions in the sampleflow will be attracted into the stream of specimen water. This streampasses through pipe 39 where colorimeter reagent is added from a supply40 with which the water is mixed in mixer 41 before entering colorimeter42. A pen recorder records the amount of chloride present in themixture.

The source of water devoid of the species of interest for supply to thecompartment 37 is conveniently the condensate in the well 25 of thesteam plant 24. The high purity condensate is first treated in thesecond electrodialysis cell 27 to remove said species before beingpassed to inlet 37. In cell 27 the polarities are such that any chlorideions present in the condensate are attracted through the membrane 31therein into a static volume 43 of water (which is replaced when itschloride ion concentration becomes too high). The stream leaving thecell 27 is thus high purity water devoid of chlorides.

Although the emphasis in the foregoing has been on testing for chloridesthe invention is equally applicable to the testing of liquids forphosphates, sulphates, nitrates etc care being taken to adjust thepolarity of the electrodes and the porosity of the membrane wherenecessary.

The concentration of positive charged species (cations) is alsopossible; however, as the cations are discharged and deposited on theplatinum electrode, in order of increasing negative potential, therewould be a risk that some species (e.g., iron copper, nickel, etc) willplate out of solution.

The invention may be adapted for the concentration of alkali metals andalkaline earths (e.g., sodium potassium, calcium, magnesium etc).

The invention may also be adapted to provide a system for de-anionisingwater (or other liquid) for use when the negative ion content of thewater is required to be low. Conversely, a static catholyte system couldbe employed to remove traces of positive ions from water flowing throughthe anode compartment.

The membranes employed in these cells may be ion-selective, but, incurrent examples microporous, i.e., neutral membranes are used and theseshould be very thin as a precaution against the transported ions beingheld up in the pores of the membrane and never reaching the specimencompartment. For this reason membranes of between 7-200 μ m; typicallyabout 100 μ m thick membranes in cellulore acetate or about 10 μ m thickin polycarbonate are preferred.

The term electroseparative has been employed because electrodialysisimplies the use of ion-selective membranes and the invention is notnecessarily limited in this respect. It is to be noted thatelectroseparative processes are very suitable for the importantmonitoring liquid flow systems described because only flow systems arelikely to avoid subsequent contamination of chloride/sulphate at thevery low concentration levels present in the input sample. There arevery low levels for example in some boiler feed water which has alreadybeen treated e.g., less than 0.5 microgram per liter.

The results of a series of comparative tests on different microporousfilters and their effect on efficiency of concentration is shown in FIG.3. These show that the pore size of the membrane is critical of a highcell efficiency is to be obtained. The optimum range of pore size isvery narrow lying between about 0.4 and 1.0 μm. The specification of themembranes was as follows:

    ______________________________________                                        Microporous Filter                                                                        Material      Membrane Thickness                                  ______________________________________                                        `Millipore` Mixed esters of                                                                             ca 150 μm                                                    cellulose                                                         `Sartorius` Cellulose nitrate                                                                           ca 90-140 μm                                     `Nuclepore` Polycarbonate film                                                                          ca 10 μm                                         ______________________________________                                    

In a series of tests designed to obtain the optimum pore size membranefor chloride migration, separate five-liter volumes of water were spikedwith ³⁶ Cl tracer and standard chloride to a chloride concentration of10 μg 1.sup.⁻¹. The solutions were then passed through the cathodecompartment at a flow rate of 28 ml min.sup.⁻¹ with an applied potentialof 100 V dc across the cell. Chlorine-36 concentrated in the anolyte wasdetermined, for calculation of cell retention efficiencies forindividual membranes.

A Carlson-Ford Cox depth filter of glass/asbestos fibre (Cox M-780) wasalso tried but these relatively thick membranes were found to absorb the³⁶ Cl tracer and the cell retention efficiencies of chloride ions werevery low in comparison with the thinner membranes.

What we claim is:
 1. An analytical apparatus for detecting the presenceof or measuring, low concentrations of ions in solution in a flowingliquid the apparatus comprising an electroseparative cell having acompartment for containing a specimen liquid substantially devoid of thespecies of interest and a compartment adapted to form part of a flowpath for a flowing sample isokinetically related to the flowing liquid,a microporous membrane separating the compartments, an electrode in eachcompartment, electrical connections for polarising the electrodesselectively to transport the ions of interest from the flowing sampleinto said compartment for containing the specimen so effecting aconcentration of the ions in the specimen compartment and a means formeasuring the concentration of the ion species of interest concentratedin the specimen compartment.
 2. An analytical apparatus as claimed inclaim 1 in which the microporous membrane is a non-ion selectivemembrane having a pore size lying in the range 0.3 μm and 0.9 μm.
 3. Ananalytical apparatus as claimed in claim 2 in which the microporousmembrane is formed of a material selected from the group, celluloseacetate ester; cellulose nitrate ester, polycarbonate.
 4. An analyticalapparatus as claimed in claim 2 in which the microporous membrane has athickness lying in the range 10-100 micrometers.
 5. In a steam boilerinstallation an analytical apparatus for monitoring the presence in theboiler feed water of ions of a species liable to promote the onset ofsome deleterious corrosive conditions, the apparatus comprising firstand second similar electro-separative cells, each cell having a samplecompartment and a specimen compartment, a neutral microporous membraneseparating each of the compartments, an electrode in each compartment, aconduit forming a flowpath for a sample of boiler feed water through thesample compartment of the first cell with the polarity of its electrodesarranged to attract ions of the species of interest through themicroporous membrane into the adjacent specimen compartment, saidadjacent specimen compartment containing a specimen of water madesubstantially devoid of said species by electro-separative treatment insaid second cell and means for analysing the species concentrated in thefirst cell.
 6. In a steam boiler installation an analytical apparatus asclaimed in claim 5 in which the specimen of water, made substantiallydevoid of ions of said species of interest is a flowing sample.
 7. In asteam boiler installation including a steam condenser an analyticalapparatus as claimed in claim 5 in which there is a condensate conduitleading condensate to the first electroseparative cell as specimenfeedwater.
 8. In a steam boiler installation as claimed in claim 5 inwhich means are provided for passing the sample of feedwater from samplecompartment the first cell to the specimen compartment of the firstcell.
 9. A method of monitoring a flowing liquid for the presence of lowconcentrations of ions of a selected species which resides in tapping anisokinetic sample from the flowing liquid, flowing the sample through achamber in which the flow is presented to a porous membrane whose poresize lies between 0.2 and 0.9 micrometers whilst maintaining anelectrical potential on the side of the membrane remote from the flowingsample and effecting electro-transport of selected ions from the flowingliquid through the membrane into a volume substantially free of saidions whereby said ions become concentrated in the volume of liquid,continuing the isokinetic sample flow until a concentration of ionssufficient for measurement has been achieved and then measuring saidconcentration.
 10. A method as claimed in claim 9 which resides inpresenting the sample flow to a porous membrane whilst maintaining anelectrical potential in the range 10 to 100 volts dc across themembrane.
 11. A method as claimed in claim 9 which resides in presentingthe sample flow to a membrane having a thickness of between 10 and 100micrometers.
 12. A method of monitoring a stream of boiler feed waterfor the presence of chloride species in concentration of the order ofless than one microgram per liter (one part per billion) which residesin subjecting an isokinetic sample flow of feed water to an electricfield polarised to transfer ions through a microporous filter membranehaving pores between 0.2 and 0.9 micrometers into a specimen volume ofliquid, and measuring the chloride concentration in said specimenvolume.